A monolithic integrated focal plane sensitive to both mm-waves (typically 94 GHz) and (typically 3-5 and 8-12 micron) IR radiation is constructed on a silicon wafer by selective anisotropic etching to fabricate microbolometer radiation sensors in a linear or two-dimensional array. Sensors intended for IR detection are coated with an IR absorbing material. Those intended for mm-wave sensing are connected to metal film antennas deposited on the surface of the silicon wafer. In this structure there is combined known silicon IC processing techniques with a rugged high-g-load-tolerant structure that permits the thermal condution losses to approach the radiative losses of the element. Of particular importance is the combining and interspersing of millimeter wave sensors with high performance infrared sensors and electronics on the same silicon chip, and fabricating in the same processing steps.
The fabrication of novel three-dimensional microelectronic devices in a semiconductor crystal, typically silicon has been accomplished by fabricating the device through many techniques including isotropic and anisotropic etching. These techniques utilize the cystalline structure of a single crystal semiconductor. An example is the Johnson et al U.S. Pat. No. 4,472,239, "Method of Making Semiconductor Device", assigned to the same assignee as the present invention. The referenced patent shows that the technique is known to manufacture micromechanical devices by etching into single crystal silicon. The citation of this patent is provided merely as background and is not deemed as prior art to the specific invention claimed in this application.
In the prior art, such as U.S. Pat. No. 3,801,949, there has been taught an infrared sensitive solid-state imaging device which is small in size and which has a two-dimensional array of IR detector elements in an integrated microcircuit. The detector array is fabricated on a single crystal silicon substrate coated with a thin layer of electrical insulating material, such as silicon dioxide or silicon nitride. Etched openings are made in the silicon beneath the insulating layer wherever a sensing element is desired for the purpose of thermally isolating the sensing elements from their surroundings. In the present invention an integrated dual-mode IR/millimeter-wave sensor array is taught. The section of the magnetic spectrum including millimeter waves and 3-5 or 8-12 micron infrared radiation is shown in FIG. 1. The mm-waves of about 94 GHz and the 3-12 micron IR are several orders of magnitude apart in frequency and devices for sensing or detecting these two categories differ substantially. It is desired to fabricate a monolithic integrated two-dimensional focal plane array which has array elements sensitive to 3-5 and/or 8-12 micron IR and elements sensitive to mm-waves. The individual integrated sensors are about 0.1 mm in size and do not effectively couple the energy from the mm-waves which are of a greater wavelength. It has been discovered that when the integrated sensor elements intended for mm-wave detection are provided with antennas (such as full wave dipoles or bow-tie type) a successful mm-wave energy coupling apparatus is achieved.